Keep only one DPB interface and rename gst_vaapi_dpb2_get_references()
to gst_vaapi_dpb_get_neighbours() so that to retrieve pictures in DPB
around the specified picture POC.
Move GstVaapiDpbMpeg2 API to a more generic version that could also be
useful to other decoders that require 2 reference pictures, e.g. VC-1.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
In P-pictures, prediction shall be made from the two most recently
decoded reference fields. However, when the first I-frame is a field,
the next field of the current picture could be a P-picture but only a
single field was decoded so far. In this case, create a dummy picture
with POC = -1 that will be used as reference.
Some VA drivers would error out if P-pictures don't have a forward
reference picture. This is true in general but not in this very specific
initial case.
